This invention relates generally to lenses of the Fresnel type and, more particularly, to techniques for chromatic correction of Fresnel lenses. A Fresnel lens has a relatively thin profile because it consists of multiple, usually annular, sections or zones separated by steps in the thickness that result in a thin profile. For a converging Fresnel lens, each of the zones can be designed to refract parallel light to approximately the same focal point. It is well known that Fresnel lenses are chromatically aberrant. In particular, each wavelength of light is refracted to a different focal point on the optical axis of the lens.
For many applications of Fresnel lenses, such as lighthouse beacons and compact magnifying glasses, chromatic aberrations are of little consequence. There are, however, other important applications of Fresnel lenses where compactness is a desirable advantage but where the ability to image over a large bandwidth is also a requirement. For example, the use of Fresnel lenses as primaries in space-based telescopes has the advantage that a large primary lens can be more easily deployed in space. If such a lens is employed for imaging astronomical or terrestrial objects, chromatic correction is highly desirable in most applications.
It is well known that a Fresnel lens can be chromatically corrected by means of a “reverse” Fresnel lens having the same optical characteristics as the one to be corrected. If the lens to be corrected is a positive lens, as used as a primary telescope optic, the reverse lens is a negative lens having the same focal length. Unfortunately, this technique, usually known as a Schupmann corrector, also requires two additional optical elements. An achromatic convergent lens is needed to bring light from the original Fresnel lens into appropriate focus on the reverse Fresnel lens. Then, because light emerging from the reverse Fresnel lens is divergent, a second conventional convergent lens is needed to focus the light from the reverse lens onto a single focal point. Thus the compactness advantage of using a Fresnel lens is diminished by the need to add a second Fresnel lens and two conventional convergent lenses.
It will be appreciated from the foregoing that there is a need for a more efficient way to provide for chromatic correction of a Fresnel lens, preferably without adding weight and optical complexity. The present invention satisfies this need.